This invention relates to apparatus for detecting the color temperature of a source of light and, more particularly, to such apparatus which produces an output signal representative of the color temperature and wherein that output signal can be used in, for example, an automatic white balance control circuit for a color television camera.
The color temperature of a source of light in, for example, a televised scene, determines the spectral characteristics of the various colors of light which is reflected from the televised object. If the light source is changed, the color temperature of the surrounding light also may change, whereby the spectral characteristics of the different color components of the light reflected from the very same object likewise will change. For example, when a source of light of relatively low color temperature is used, measurements of the spectral characteristics of light which is reflected from, for example, a white reference, indicate that the relative energy, or intensity, of light in the red passband is greater than the relative energy, or intensity, of light in the blue passband. Conversely, when the light source admits of a higher color temperature, the relative energy of light in the blue passband exceeds that of the light in the red passband.
In color television applications, if the image of a white object is projected to a color television camera, the resultant video picture of that object also must be white. For pure white objects, the red, blue and green color signal components produced by the color camera should be present in the ratio of 1:1:1. Of course, the color signal components which are produced by the camera are dependent upon the relative intensities, or energies, of the different colors of light which are received by that camera. However, as noted above, these intensities are dependent upon the color temperature of the light source which is used to illuminate the televised object. Thus, if the television picture of a pure white object appears as a pure white image for one light source, a change in the color temperature of that source, as by replacing it with another light source, will result in a television picture which does not appear as a pure white object. This is because, as the color temperature of the light source is changed, the intensities of the red, blue and green light images which are reflected to the television camera no longer will be present in the desired ratio of 1:1:1. For example, for a light source of a relatively lower color temperature, the intensity of red light reflected from a pure white object may exceed the intensity of green light which, in turn, may exceed the intensity of blue light. Conversely, for a color source of relatively higher color temperature, the intensity of blue light in the reflected light image of a white object may exceed that of the green light which, in turn, may exceed that of the red light.
To account for the effect of a change in the color temperature of the light source, it is conventional to provide a color television camera with a so-called white balance adjustment. This serves to adjust the magnitude of the respective color signals, e.g. the red, blue and green color signals, so that they will exhibit the proper ratio even though the ratio of red to blue to green light intensities received by the camera is not 1:1:1. Typically, such white balance adjustment is attained by using a white reference as the televised object, and then manually adjusting the respective color signals which are produced by the camera in response to that object. If the color temperature of the light source is known, this manual adjustment is not overly difficult. Unfortunately, it often is not a simple task to determine the color temperature. Also, it is desirable to ascertain whether the color temperature of a light source varies over a period of time. If color temperature can be detected or measured accurately, the color video signal components produced by a color television camera can be controlled in response to the detected color temperature so as to automatically attain a white balance adjustment.